It is well known that dihydroxypiperidines may be dehydrated to produce dihyropyridines by the use of an acid or an anhydride. U.S. Pat. No. 4,692,184 (Lee, 9/87) discloses several agents and the conditions under which they were used to produce various fluorinated dihydropyridinedicarboxylates which were in turn used to produce herbicidal fluorinated pyridinedicarboxylates, including dithiopyr. Example GG therein discloses the dehydration of dimethyl 2,6-bis(trifluoromethyl)-2,6-dihydroxy-4-(2-methylpropyl)-3,5-piperidinedi carboxylate using concentrated sulfuric acid in methylene chloride at &lt;10 C. In this example, sulfuric acid is used at the molar ratio of 32.6 moles per mole of dihydroxypiperidine. Example HH of '184 discloses dehydration of trans-diethyl 2,6-bis(difluoromethyl)-2,6-dihydroxy-4-(2-methylpropyl)-3,5-piperidinedic arboxylate using excess trifluoroacetic anhydride at ambient temperatures up to 36.degree. C. Example II of '184 discloses dehydration of diethyl 2,6-bis(trifluoromethyl)-2,6-dihydroxy-4-(2-thienyl)-3,5-piperidinedicarbo xylate using p-toluenesulfonic acid in toluene under reflux conditions.
In U.S. Pat. No. 4,908,057 (Chupp 3/90), the reaction disclosed in Example GG of '184 is reported as being performed in toluene.
In U.S. Pat. No. 5,051,512 (Baysdon, 9/91), a process for the dehydration of the thiol ester analogs, dihydroxypiperidinedicarbothioates, is disclosed. The dehydration agent is hydrogen chloride or aqueous hydrochloric acid. The reaction is run at about 80.degree. C., preferably with no solvent present.
However, none of the previously reported dehydration procedures provide a high-yielding, cost-effective process with minimal by-product formation and minimal waste stream production for the conversion of dihydroxypiperidinedicarboxylates to dihydropyridinedicarboxylates. It is an object of the present invention to provide such a process. It is a further object of the present invention to provide such a process requiring a reduced amount of dehydration agent.